We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back-arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age-depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems.

/pdf/age-spreading-rates-and-spreading-asymmetry-of-the-world-s-5bj3otyvfc.pdf

Age, spreading rates, and spreading asymmetry of the world's ocean crust

/pdf/towards-the-standardization-of-sequence-stratigraphy-b61e2b7dm7.pdf

Towards the standardization of sequence stratigraphy

The sudden, widespread glaciation of Antarctica and the associated shift towards colder temperatures at the Eocene/Oligocene boundary (approximately 34 million years ago) (refs 1-4) is one of the most fundamental reorganizations of global climate known in the geologic record. The glaciation of Antarctica has hitherto been thought to result from the tectonic opening of Southern Ocean gateways, which enabled the formation of the Antarctic Circumpolar Current and the subsequent thermal isolation of the Antarctic continent. Here we simulate the glacial inception and early growth of the East Antarctic Ice Sheet using a general circulation model with coupled components for atmosphere, ocean, ice sheet and sediment, and which incorporates palaeogeography, greenhouse gas, changing orbital parameters, and varying ocean heat transport. In our model, declining Cenozoic CO2 first leads to the formation of small, highly dynamic ice caps on high Antarctic plateaux. At a later time, a CO2 threshold is crossed, initiating ice-sheet height/mass-balance feedbacks that cause the ice caps to expand rapidly with large orbital variations, eventually coalescing into a continental-scale East Antarctic Ice Sheet. According to our simulation the opening of Southern Ocean gateways plays a secondary role in this transition, relative to CO2 concentration.

/pdf/rapid-cenozoic-glaciation-of-antarctica-induced-by-declining-4yuos133vq.pdf

Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO2.

Clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones. Based on a critical appraisal of the geophysical and geological literature, 26 parameters are estimated for each of 39 modern subduction zones. To isolate causal relationships among these parameters, multivariate analysis is applied to this data set. This analysis yields empirical quantitative relations that predict strain regime and strike-slip faulting in the overriding plate, maximum earthquake magnitude, Benioff zone length, slab dip, arc-trench gap, and maximum trench depth. Excellent correlation is found between length of the Benioff zone and the product of convergence rate and age of the downgoing slab. This relationship is consistent with the conductive heating model of Molnar et al. (1979), if the model is modified in one respect. The rate of heating of the slab is not constant; it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate. The structural style in the overriding plate is determined by its stress state. Though the stress state of overriding plates cannot be quantified, one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional (class 1, back-arc spreading) to strongly compressional (class 7, active folding and thrusting). This analysis indicates that strain class is probably determined by a linear combination of convergence rate, slab age, and shallow slab dip. Interplate coupling, controlled by convergence rate and slab age, is an important control on strain regime and the primary control on earthquake magnitude. Arc-parallel strike-slip faulting is a common feature of convergent margins, forming a forearc sliver between the strike-slip fault and trench. Optimum conditions for the development of forearc slivers are oblique convergence, a compressional environment, and a continental overriding plate. The primary factor controlling presence of strike-slip faulting is coupling; strongly oblique convergence is not required. The rate of strike-slip faulting is affected by both convergence obliquity and convergence rate. Maximum trench depth is a response to flexure of the underthrusting plate. The amount of flexural deflection at the trench depends on the vertical component of slab pull force, which is very sensitive to slab age and shallow slab dip. Shallow slab dip conforms to the cross-sectional shape of the overriding plate, which is controlled by width of the accretionary prism and duration of subduction. Deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow. Much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike-slip faulting and strain regime. Empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones.

Relations among subduction parameters

[1] We present a new global model for Recent plate velocities, REVEL, describing the relative velocities of 19 plates and continental blocks. The model is derived from publicly available space geodetic (primarily GPS) data for the period 1993–2000. We include an independent and rigorous estimate for GPS velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates. The velocity fields for North America, Eurasia, and Antarctica clearly show the effects of glacial isostatic adjustment, and Australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field. Two thirds of tested plate pairs agree with the NUVEL-1A geologic (3 Myr average) velocities within uncertainties. Three plate pairs (Caribbean–North America, Caribbean–South America, and North America–Pacific) exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in NUVEL-1A due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities. Most other differences probably reflect real velocity changes over the last few million years. Several plate pairs (Arabia–Eurasia, Arabia–Nubia, Eurasia–India) move more slowly than the 3 Myr NUVEL-1A average, perhaps reflecting long-term deceleration associated with continental collision. Several other plate pairs, including Nazca–Pacific, Nazca–South America and Nubia–South America, are experiencing slowing that began ∼25 Ma, the beginning of the current phase of Andean crustal shortening.

/pdf/revel-a-model-for-recent-plate-velocities-from-space-geodesy-18oq1lu9ey.pdf

REVEL: A model for Recent plate velocities from space geodesy

The structure of the crust and upper mantle of the Central Volcanic Region, New Zealand, are deduced from long-range seismic refraction and wide-angle seismic reflection data. Three shots were detonated in the Bay of Plenty and one in Lake Taupo; seismographs were located at approximately 10 km intervals between these shots. The geometry of the experiment was such that there was a partially reversed coverage for the crustal phases, but correlatable upper mantle phases were only recorded from the Bay of Plenty shots. A nondipping, plane-layer model to account for the refraction observations consists of a 15 ± 2 km thick crust of seismic velocity 5.5–6.15 km/s, underlain by an upper mantle with an anomalously low velocity of 7.5 ± 0.2 km/s. Wide-angle upper mantle reflections were also recorded from the Lake Taupo shot; a simple T2 versus X2 analysis of these reflections gives a depth of about 16±1 km to the upper mantle with a root-mean-square (r.m.s.) velocity for the crust of 5.94 ± 0.2 km/s. Th...

A seismic investigation of the crustal and upper mantle structure within the Central Volcanic Region of New Zealand

The Alpine Fault of central South Island New Zealand, can be tracked with seismic reflection methods to depths of ∼35 km as a listric-shaped surface with strong reflectivity. Maximum dips of the surface are ∼60 degrees at 15 km depth and the dip then lessens with depth until the reflectivity is sub-horizontal at ∼35 km. Wide-angle seismic methods are used to show that the P-wave velocities of the rocks are up to 10% less than normal in the zone above the fault surface. In cross-section this low-velocity Alpine Fault Zone is elongate, sits above the fault surface, and has dimensions roughly 45 by 20 km. A magnetotelluric study shows a low-resistivity anomaly that is roughly coincident with the zone of low seismic velocity. A straightforward interpretation is that both the electrical and seismic anomalies are caused by interconnected fluids at lithostatic pressure. The inference of fluids in the lower crust is supported by an attribute analysis of seismic reflections on specific shot gathers where the Alpine Fault reflections can unequivocally be identified. We reference both the amplitude and phase of the fault-zone reflections to the distinctive side-swipe reflections generated at the far shore of Lake Pukaki. High reflection coefficients of ∼0.25 are estimated for the Alpine Fault reflections, which may require both anisotropy and fluid to explain. We interpret the source of water to be metamorphic dewatering of the schist-greywacke rocks that thicken into the orogen. A detachment surface along which the greywacke-schist rocks are obducted is recognised as a zone of strong reflectivity on an 80-km-long, unmigrated seismic reflection section. This zone of strong reflectivity, which apparently merges into the Alpine Fault reflections, does not correlate with depth to the Moho but rather with the boundary between the base of the schist-greywacke rocks (Vp ∼6-6.2 krn/s) and the lower crust (Vp ∼7-7.2 km/s). We interpret the strong reflectivity on this boundary as being due to a shear fabric. Both geological and geophysical observations imply deformation in the lower and mid-crust and mantle that appears to be caused by a combination of ductile and brittle behaviour, with no evidence of lithospheric flexure. We interpret the Alpine Fault Zone as a profoundly hot, wet, and weak region of continental crust.

/pdf/geophysical-exploration-and-dynamics-of-the-alpine-fault-4cngd6c0kd.pdf

Geophysical exploration and dynamics of the Alpine Fault Zone

Summary. The crustal and upper mantle structure of the northwestern North lsland of New Zealand is derived from the results of a seismic refraction experiment; shots were fired at the ends and middle of a 575 km-long line extending from Lake Taupo to Cape Reinga. The principal finding from the experiment is that the crust is 25 f 2 km thick, and is underlain by what is interpreted to be an upper mantle of seismic velocity 7.6 f 0.1 km s-', that increases to 7.9 km s-l at a depth of about 45 km. Crustal seismic velocities vary between 5.3 and 6.36 km s-l with an average value of 6.04 km s-'. There are close geophysical and geological similarities between the northwestern North Island of New Zealand and the Basin and Range province of the western United States. In particular, the conditions of low upper-mantle seismic velocities, thin crust with respect to surface elevation, and high heatflow (70-100 mW m-') observed in these two areas can be ascribed to their respective positions behind an active convergent margin for about the past 20 Myr.

Crustal and upper mantle structure of the northwestern North Island, New Zealand, from seismic refraction data

Compression of the entire continental lithosphere is considered using two-dimensional numerical models, in order to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages. The models are based on the central section of New Zealand Southern Alps, where continental collision has occurred along the Alpine Fault since about 7 Ma. They incorporate brittle-elastic-ductile rheology, heat transfer, surface processes, and fault localisation. The models are compared to the surface relief, the GPS convergence velocity, the measured electrical conductivity, and the geometry of the crustal root imaged from seismic velocity measurements. The crustal deformation is characterized by localized uplift at the plate boundary (Alpine Fault) and by two secondary zones of faulting. One is located about 60 to 80 km east of the Alpine Fault, at the start of upper crust bending (or tilting), and the other is located about 100 to 130 km east of the Alpine Fault as a result from shear strain propagating to the surface through the ductile lower crust. The observed asymmetric shape of the crustal root is best reproduced for a mantle lithosphere strength of the order of 200 MPa, and an intermediate rate of strain-softening. A lower strength of the mantle lithosphere can produce symmetric thickening, but provides an amplitude of the crustal root too small when compared to observations. The observed 20 km offset between the maximum in surface relief and the crustal root was not satisfactorily reproduced. This offset is most likely due to the three dimensionality of oblique collision in New Zealand Southern Alps.

/pdf/numerical-models-of-lithospheric-deformation-forming-the-1ccm3effn6.pdf

Numerical models of lithospheric deformation forming the Southern Alps of New Zealand

Initial results from a seismic experiment in the central South Island, New Zealand, have imaged a 40 ± 5° southeast‐dipping zone at a depth of c. 22 km beneath the Mt Cook village. It is speculated that this reflector represents the down‐dip extension of the Alpine Fault Zone.

/pdf/crustal-reflections-from-the-alpine-fault-zone-south-island-kgxbxmxkpl.pdf

Fred Davey

Papers

A seismic investigation of the crustal and upper mantle structure within the Central Volcanic Region of New Zealand

Geophysical exploration and dynamics of the Alpine Fault Zone

Crustal and upper mantle structure of the northwestern North Island, New Zealand, from seismic refraction data

Numerical models of lithospheric deformation forming the Southern Alps of New Zealand

Crustal reflections from the Alpine Fault zone, South Island, New Zealand